Powders composed of fine particles are utilized in one or more states of many industrial processes. Examples of such powders include food, pharmaceuticals, abrasives, pigments, plastics and magnetic coating materials. It is frequently desirable to measure the sizes of the particles in a powder. Another application of particle size measurement systems is for environmental analysis of airborne particulates such as smoke from industrial smoke stacks. Prior art techniques for measuring particle sizes have included sifting through meshes of various sizes, impingement of particles on piezoelectric devices, instruments wherein the particles are suspended in a liquid, light and electron microscopy, scattered light intensity measurements, light blockage measurements and instruments which utilize Fraunhofer diffraction patterns. Prior art particle size measurement systems have been restricted by a variety of disadvantages. Some have an inability to measure particles in the micron and submicron size range, others lack resolution between particles of different sizes and still others have an inability to measure a variety of particle types. For example, the instrument wherein particles are suspended in a liquid for measurement by Coulter principle or sedimentation is useless for particles that dissolve in or chemically react with that liquid.
A technique for particle size measurement utilizing a time-of-flight technique is disclosed in U.S. Pat. No. 3,854,321 issued Dec. 17, 1974 to Dahneke. An aerosol containing the particles to be measured is accelerated through a nozzle and is injected through two spaced-apart beams of light in a vacuum chamber. As a particle passes through each light beam, light scattering occurs. A detector receives a scattered light pulse from the first beam and from the second beam for each particle. The time delay between pulses represents particle velocity which is directly related to particle size. The time-of-flight technique for particle size measurement is also disclosed by D. B. Blackford et al "Particle Size Analysis With An Aerodynamic Particle Sizer", Proceedings of the 11th Annual Powder and Bulk Solids Conference, Rosemont, Ill., pp. 615-623, May 12-15, 1986 and J. K. Argarwal et al "An Instrument For Real Time Aerodynamic Particle Size Analysis Using Laser Velocimetry", Proceedings of The Inhalation, Toxicology and Technology Symposium, ed. by Basil K. J. Leong, Ann Arbor Science Publishers, 1981, pp. 207-231. The output of such an instrument is typically a distribution of the number of particles measured at each particle size over a range of sizes.
The time of fliqht technique has in the past suffered from various disadvantages. It is desirable in the time-of-flight technique to utilize a supersonic flow of particles through the measurement region. When the flow is supersonic, the particle velocity is independent of pressure variations in the air source. However, in practical nozzle configurations, supersonic flow occurs only over a distance of 2 to 3 millimeters from the end of the nozzle. Thus, the two light beams utilized to measure particle velocity must be spaced on the order of about one millimeter. Conventional beam splitting techniques utilizing partially transmissive mirrors are impractical and difficult to align for such closely-spaced beams. A calcite plate has also been utilized for splitting a beam into two closely-spaced components (see Argarwal et al publication referenced hereinabove). However, the calcite plate produces output beams with different polarizations. This is undesirable since light scattering by particles will be different for the two beam polarizations.
Another difficulty with prior art time-of-flight particle size measurement systems has been detection of the two closely-spaced light beams. It is desirable to separately detect the two beams so that start and stop pulses are automatically identified. However, it is physically difficult to position separate detectors only one millimeter apart. Optical fibers can be utilized to conduct the light beams to remotely located detectors. However, a significant portion of the scattered light is reflected from the end face of each optical fiber.
A further difficulty with prior art time-of flight particle size measurement systems relates to the injection of particles through the two light beams for measurement. Samples to be measured are often in the form of a powder sample which is an agglomeration of particles that are usually in clusters due to attraction by electrostatic forces. In order to accurately measure the particles using the time-of-flight technique, the clusters must be separated into individual particles and carried by air, one at a time, as a particle stream through the measurement chamber. Clearly, if clusters of particles pass through the measurement chamber, individual particles cannot be measured. In addition, particles in an aerosol tend to stick to the walls of any container through which they pass.
It is a general object of the present invention to provide an improved particle size measurement system.
It is another object of the present invention to provide apparatus for generating closely-spaced first and second light beams in an image plane, each light beam having a relatively thin, elongated cross sectional shape.
It is yet another object of the present invention to provide apparatus for generating closely spaced first and second light beams that have the same polarization.
It is still another object of the present invention to provide apparatus for generating closely spaced first and second light beams wherein the spacing between light beams can be accurately controlled.
It is still another object of the present invention to provide a particle sizing system incorporating the above features.